Nothing happens when a server is stopped with /stop: your stop message must contain one of planned-shutdown-messages (see shutdown-message in bukkit.yml or the message your restart plugin uses).
"config.yml is not valid: line N ..." on startup: VelorFailover stays idle. Fix the line and run /velorfailover reload.
Players are not redirected: check /vf status; both servers must show online when the crashed one is kicked from, the fallback must be registered in velocity.toml, and the route must exist in fallback-routes.
A switch command does nothing: another plugin may own that name (see the startup warning) or the target is not in velocity.toml.
